Number 171 (Story #2), April 1, 1994 by Phillip F. Schewe and Ben Stein PUNCTUATED EQUILIBRIUM is the name for Harvard paleontologist Stephen Jay Gould's theory that evolution comes about not just by gradual steps but sometimes because of catastrophic events, such as meteor impacts. Kim Sneppen of the Niels Bohr Institute in Denmark, speaking at the APS meeting, believes that evolutionary bursts may also occur because of the dynamics of ecological systems themselves. Sneppen and his colleague Per Bak of Brookhaven have proposed a model in which biological species can exhibit "self-organized criticality," according to which some systems, such as sand dunes or geological faults, can accommodate the gradual addition of energy or stress or small increments of matter until a certain threshold is crossed, after which a catastrophic reordering takes place, such as an earthquake or avalanche. Sneppin's model, employing various rates of mutation and interactions among species, seems to forecast such avalanches for biological systems. (Science News, 26 March 1994.)